1. Field of the Invention
This invention is concerned with the biasing of power transistors and is more specifically concerned with controlling the current gain of a saturated power transistor.
2. Prior Art
The power transistor in a converter circuit is normally periodically switched from its off condition into a saturated mode of operation. The magnitude of current flow through the saturated transistor is largely determined by the demands of the load connected to or coupled to the power transistor. This gives rise to a specific problem in controlling the base drive current to the saturated power transistor. In its saturated condition the collector current, which is responsive to the demands of the load, may vary but the base drive current normally remains constant. Hence, if the collector current becomes low, excessive stored charge will accumulate in the saturated transistor making the subsequent switching transition interval from conduction to nonconduction increase in time. On the other hand, the collector current may increase to a magnitude so high than the transistor may drop out of its saturated condition.
The prior art has attempted to solve this problem by use of transformer feedback. A particular example of this solution is disclosed in U.S. Pat. No. 3,735,235, issued to B. H. Hamilton et al on May 22, 1973. This patent discloses a rectifier circuit utilizing synchronous rectifying transistors. The transistors are periodically driven into saturation to rectify the applied input AC signals. To provide the proper base drive current when the transistor is saturated, a regenerative feedback base drive arrangement is utilized to drive the transistors into saturation. The regenerative feedback base drive arrangement comprises a feedback transformer which has individual transformer windings coupled to the base and collector of the rectifying transistors so as to produce a controlled base drive current that is proportional to the collector current. This proportionality is achieved through the regenerative action of the transformer which transforms the collector current of the rectifying transistor into the base drive current. While this arrangement satisfactorily maintains the rectifying power transistors in their saturated state, transformer-type feedbacks tend to be expensive and space consuming. In addition, due to the operative nature of the transformer, the power transistor being controlled tends to be latched either into a conducting or a nonconducting condition.